Evolution of density compensated fronts in simulated ocean mixed layers

Abstract:

Observations within the ocean surface mixed layer indicate a tendency for temperature gradients to form that are compensated for their effect on density by salinity gradients. These density compensated fronts tend to occur in the absence of strong surface forcing and thus weak vertical mixing. Observations show that density compensated fronts are quickly erased by surface cooling events. The presence of density compensated gradients in the surface mixed layer, however, are not well represented in regional and global ocean circulation model predictions. In these models, subgrid-scale processes are parameterized with minimal ability to represent double diffusion. Recent advances in parameterizations have been developed to model the re-stratification of the mixed layer by sub-mesoscale eddies. These ageostrophic dynamics can lead to long filaments that are governed by process on length scales from 100 m to 10 km and time scales near a day. The impact of these processes in model physics on density compensated fronts is unclear. To improve our understanding of compensated front evolution in the ocean, three different mixing schemes are tested to evaluate the creation of horizontally density compensated gradients in model simulations. One scheme extracts potential energy of ocean fronts for mixing dependent on horizontal and vertical buoyancy gradients, mixed layer depth, and inertial period. The other two schemes mix temperature and salinity horizontally dependent on the buoyancy gradient. All schemes provide a three dimensional approach to mixing that differentiates the horizontal eddy diffusion of temperature and salinity.